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Galaxies are believed to be embedded in a “cosmic web”, the three-dimensional cellular foam arrangement of matter in the Universe predicted by the standard cold dark matter cosmological paradigm [1]. Most of the baryons do not reside in galaxies, but are spread along this web in highly ionized gaseous medium [2] that is too rarefied to form stars. While intergalactic gas may have been observed as absorption features in the spectra of background sources [3], direct constraints on the three-dimensional properties and morphology of the cosmic web are still missing. Limited by the rarity of bright background sources, absorption studies are only able to provide one-dimensional skewers of the cosmic web that are typically separated by several tens of Mpc. Direct detection of intergalactic gas in emission would instead provide a full three-dimensional image significantly improving our understanding of cosmological structure formation and the cycle of baryons in and out of galaxies.

Despite the predicted low surface brightness, there have been attempts to detect the cosmic web in Lyman α emission, e.g., by means of low-resolution spectroscopy [4] to search blindly for fluorescence generated by optically thick gas illuminated by the cosmic UV background [5]. Achieving a very deep flux limit of 8x10-20erg s-1 cm-2 arcsec-2, these observations failed to reveal the cosmic web. Positive fluctuations in the ionizing background may be used to increase the expected fluorescent signal [6]. In a pilot survey obtained in 2010 using a custom-built, narrow-band (NB) filter on the VLT-FORS we demonstrated indeed that bright quasars can, like a flashlight, “illuminate” the densest knots in the surrounding cosmic web and boost fluorescent Lyman α emission to detectable levels [7]. In this survey we found several compact ”dark galaxies” and extended nebulae (up to 65 physical kpc) around star forming galaxies, but none of them extending on intergalactic scales. Following the same experiment, we have initiated in 2012 a NB imaging campaign on Keck/LRISb centered on z~2 bright quasars and we have reported in a recent Nature letter [8] the first result of this new imaging survey.

Figure 1 : Processed and combined images of the field surrounding the quasar UM287. Each image is 2 arcmin on a side and the quasar is located at the center. In the narrow-band (NB3985) image (panel 'a'), which is tuned to the Lyman α line of the systemic redshift for UM287, one identifies very extended (≈ 55 arcsec across) emission – that we named "Slug Nebula". The deep V-band image (panel 'b') does not show any extended emission associated with UM287. This requires the Slug Nebula to be line-emission, and we identify it as Lyman α at the redshift of the quasar.

On November 12 and 13, 2012, we imaged the field of the quasar UM 287 with a custom NB filter tuned to Lyman α at z = 2.279 inserted into the Keck/LRISb
camera on the 10m Keck-I telescope. We acquired 10 hours of integration in a series of dithered, 1200s exposures in clear conditions. In parallel (enabled by a dichroic), we obtained broad-band V images with the LRISr camera. Figure 1 presents the processed and combined images, centered on UM287. The V -band image is very deep and hundreds of compact sources are present in the field. We expect the majority of these are background galaxies, unrelated to the system. In the NB3985 image, however, one identifies a very extended source originating near the quasar with a projected size of about 1 arcmin (500kpc physical or 1.6 Mpc co-moving). We will refer to this extended emission as the ”Slug Nebula” in the reminder of this article. Within the nebula, very few sources are identified in the broad-band images nor is any extended emission observed. This requires the narrow-band light to be line-emission, and we identify it as Lyman α at the redshift of UM287.

Figure 2 : Lyman α image of the Slug Nebula. We subtracted from the NB image the continuum contribution estimated from the broad-band images. The location of the quasar UM287 is labeled with the letter “a”. The color map and the contours indicates, respectively, the Lyman α surface brightness and the signal-to-noise ratio (S/N) per arcsec2 aperture. The extended emission spans a projected angular size of ≈ 55 arcsec (about 460 physical kpc), measured from the 2σ (~10−18 erg s-1 cm-2 arcsec-2) contours. Object “b” is an optically faint (g~23AB) quasar at the same redshift of UM287. The Nebula appears broadly filamentary and asymmetric, extending mostly on the eastern side of quasar “a” up to a projected distance of about 35 arcsec (~285 physical kpc) measured from the 2σ isophotal.

Figure 2 presents the NB3985 image, continuum subtracted using standard techniques [8]. One identifies several compact sources including UM287 (labeled “a” in the figure) with excess Lyman α emission. The second brightest compact emitter (indicated by the letter “b”) is an optically faint (g~22 AB) quasar at the same redshift of UM287. The image is dominated, however, by the filamentary and asymmetric Slug Nebula. Although Lyman α nebulae extending up to about 250 kpc have been previously detected [9-13], the Slug Nebula represents so far a unique system as we show in Figure 3: with a size of about 55” or 460 physical kpc, it extends well beyond the virial radius of any plausible dark matter halo associated with UM287. Indeed, in order to be fully contained within the virial radius of a dark matter halo centered on UM287, the quasar host halo should have a total halo mass of 1013.5 Msun. This is ten times larger than the typical value associated with radio-quiet quasars (1012.5 Msun, see [8] for discussion) and it would make the host halo of UM287 one of the largest know at z > 2. However, this possibility is clearly excluded by the absence of an excess of Lyman α emitting galaxies around UM287 compared to other radio-quiet quasars. Our analysis of the galaxy distribution around UM287 suggests instead that this quasar is residing in a typical or under-dense environment for radio-quiet quasars and that its total halo mass therefore does not exceed 1012.5 Msun. Differently from any previous detection, the Slug Nebula is therefore the first possible image of intergalactic gas at z > 2 extending beyond any individual, associated dark matter halo. The rarity of these systems may be explained by the combination of anisotropic emission from the quasars (typically only about 40% of the solid angle around a bright, high-redshift quasar is unobstructed [14]), the anisotropic distribution of dense filaments and light travel effects that, for quasar ages younger than a few Myr, further limit the possible ”illuminated” volume.

Figure 3 : Luminosity-size relations for previously detected, bright Lyman α nebulae and the Slug Nebula around the quasar UM287. The plot includes nebulae surrounding AGN and Lyman α blobs (LAB). The dashed line indicates the virial diameter of a dark matter halo with total mass M ~ 1012.5 Msun, the typical host of radio-quiet quasars including UM287, as confirmed by the analysis of the galaxy overdensity in our field. The Slug Nebula, differently from any previous detection, extends on Intergalactic Medium scales that are well beyond any possible associated dark matter halo. Note that, even if we restrict the size measurement of the UM287 Nebula to the 4 × 10−18 erg s-1 cm-2 arcsec-2 isophotal to be comparable with the majority of the previous surveys, the measured apparent size of the Slug Nebula will be reduced only by about 20%.

In order to constrain the physical properties of this, so far, unique system, we use a set of Lyman α radiative transfer calculations [15] combined with hydrodynamical simulation of cosmological structure formation around a quasar halo host similar to UM287. We consider two possible, extreme scenarios for the Lyman α emission mechanism of the intergalactic gas associated with the Slug Nebula: a) the gas is mostly ionized and the Lyman α emission is mainly produced by hydrogen recombinations. b) the gas is mostly neutral and the emission is mainly due to scattering of the Lyman α and continuum photons produced by the quasar Broad Line Region (BLR). In both cases, we performed a full three dimensional Lyman α radiative transfer calculation including gas temperature and velocity field effects on Lyman α scattering within the Nebula. The models are used to obtain the scaling relations between the observable Lyman α surface brightness from the intergalactic gas surrounding the quasar and the hydrogen column densities. Through these relations, we converted the observed SB into an estimated gas column density for the two extreme scenarios. Note that the estimated column densities for case ”a” are degenerate with the ionized gas clumping factor (C =2

>/2, where n is the electron density) below the simulation resolution scale, ranging from ~10 proper kpc for diffuse intergalactic gas to ~100 pc for the densest regions within galaxies.

The results, using the observed BLR Lyman α luminosity and C = 1, are presented in
Fig.4. The observed Lyman α emission from the intergalactic gas associated with the Slug Nebula requires very large column density of ”cold” (T < 5 x 104 K) gas to be matched by current simulations. The implied total, cold gas mass ”illuminated” by the quasar is Mgas ~1011.4±0.6 Msun for the ”mostly neutral” case (”b”) and Mgas ~1012±0.5Msun for the ”mostly ionized” case (”a”) and C = 1. Note that the total estimated mass for the case ”a” scales as C1/2. For comparison, a typical simulated filament in our cosmological simulation of structure formation with size and morphology similar to the Slug Nebula around a similar halo has a total gas mass of about 1011.3 Msun, but only about 15% of this gas is ”cold” (T < 5x104 K), i.e. 1010.5 Msun and therefore able to emit substantial Lyman α emission. These estimates are consistent with other recent, grid-based hydrodynamical simulations of structure formation [16].

Figure 4 : Inferred hydrogen column densities associated with the Slug Nebula. We have converted the observed Lyman α Surface Brightness into gas column densities using a set of scaling relations obtained with detailed radiative transfer simulations. We have explored two extreme cases: a) the gas is mostly ionized by the quasar radiation (panel “a”), b) the gas is mostly neutral (panel “b”). Two circular regions with a diameter of 7 arcsec (~ 8 times the seeing radius) have been masked at the location of the quasars (black circles). The inferred hydrogen column density in panel “a” scales as C−1/2, where C is the gas clumping factor below a spatial length of up to about 10 physical kpc at moderate overdensities (less than about 40 times the mean density of the Universe at z~2). The implied column densities and gas masses, in both cases, are at least a factor of ten larger than what is typically observed within cosmological simulations around massive haloes, suggesting, e.g., that a large number of small clumps within the diffuse Intergalactic medium may be missing within current numerical models.

How one can explain the large differences between the estimated, cold gas mass of the Slug Nebula and the available amount of cold gas predicted by numerical simulations on similar scales? The Slug Nebula seems to point in the direction of a second, fainter quasar companion of UM287. However, because of the large distance from UM287 -- at least 200 proper kpc and up to 4 proper Mpc considering the 1σ redshift error, and the morphology of the Nebula we can exclude that the UM287 Nebula is the result of tidal interaction due to a merging event between the two quasar hosts. Indeed, such a large separation would imply that any possible encounter between the two quasars is likely a high velocity interaction or an encounter with large impact parameter. We note that it is not impossible but extremely difficult to produce a long and massive tidal tail during a ”fast” encounter but the amount of gas stripped by the quasar host galaxies in the best scenario would likely be a very small fraction (< 10%) of its total ISM and certainly cannot account for the total amount of gas detected in the Nebula. Irrespective of the details of the
possible interaction between the two quasar host galaxies, any resulting, long tidal tail would be very thin with sizes of the order of few kpc or less while the observed Nebula has a thickness of at least 100 physical kpc in its widest point.

Similarly, it would be very difficult to explain the properties of the Nebula assuming a galactic gas outflow origin produced by possible quasar feedback events. Indeed, although radio-quiet quasar outflows are highly unconstrained from current observations and poorly understood theoretically, the large size of the Nebula, extending well beyond the virial radius of the quasar host halo, would require a high velocity outflow that is incompatible with the “cold” temperature of the gas required by the Lyman α emission. A recent spectroscopic follow-up (Cantalupo et al., in preparation) provides additional evidences that the Nebula is kinematically quiet and therefore that it cannot be generated by “quasar feedback”. The size, morphology and kinematical properties of the gas are instead broadly consistent with our expectations from a filament of the “cosmic web”.

How one can then reconcile the intergalactic nature of the Slug Nebula with the large mass discrepancy with intergalactic gas simulations? One possibility is to assume that the simulations are not resolving a large population of small, cold gas clumps within the low-density Intergalactic medium that are illuminated and ionized by the intense radiation of the quasar. In this case, an extremely high clumping factor, namely C ~1000, on scales below few kpc would be required in order to explain the large luminosity of the Slug Nebula with the cold gas mass within the intergalactic filaments predicted by the simulations. On the other hand, if some physical process that is not fully captured by current grid-based simulations increases the fraction of cold gas around the quasar, e.g. a proper treatment of metal mixing, a smaller clumping factor may be required. In the extreme – and rather unrealistic - case that all the hot gas is turned into a cold phase, the required clumping factor would be C ~20. Even if the gas is not ionized by the quasar (case ”b” above), the simulations are able to reproduce the observed mass only if a substantial amount of hot gas is converted into a cold phase. Incidentally, this is exactly the same result found comparing the properties of Lyman α absorption systems around a large statistical sample of quasars with simulations [17].

The discovery of the Slug Nebula represents both a unique laboratory and a challenge for
our knowledge of cosmological structure formation on Intergalactic scales around massive haloes. On one hand, it provides a fundamental confirmation that specifically designed, deep narrow-band surveys centered on bright quasars are able to provide - for the first time - an image of cosmic gas on intergalactic scales. The rarity of such detection however, may imply that several conditions regarding, e.g. the geometry of the quasar "illumination" are met arguing for the necessity of a very large sample of quasars. On the other hand, our observation indicates that current models of cosmological structure formation (at least numerical methods based on Adaptive Mesh Refinement
algorithms) are far from providing an accurate picture of the gas properties - not only within galaxies - but also for diffuse Intergalactic gas within several hundreds of physical kpc from massive haloes at z ~2. In particular, the size and luminosity of the Slug Nebula suggest that a large population of cold, sub-kpc scale clumps may be present within the diffuse Intergalactic medium in proximity of quasars. Proper modeling of this gas phase will require a new generation of numerical models that are able - simultaneously - to spatially resolve these small intergalactic clumps within large simulation boxes, treat the multiphase nature of this gas and its interaction with galaxies and quasars.

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